Plasmodium is a genus of parasitic protozoa. Infection with this genus is known as malaria. The parasite always has two hosts in its life cycle: a mosquito vector and a vertebrate host. At least ten species infect humans including P. falciparum, P. vivax, P. malariae, and P. ovale. Malaria is an infectious disease that is widespread in tropical and subtropical regions. Malaria represents a threat to survival of men, women and children. It infects between 300 and 500 million people every year and causes between one and three million deaths annually, mostly among young children in Sub-Saharan Africa.
Malaria is one of the most common infectious diseases and an enormous public-health problem. The disease is caused by protozoan parasites of the genus Plasmodium. The most serious forms of the disease are caused by Plasmodium falciparum and Plasmodium vivax, but other related species (Plasmodium ovale and Plasmodium malariae) can also infect humans. Determining the infectious species helps determine the course of treatment for the patient.
The preferred and most reliable diagnosis of malaria is microscopic examination of blood films because each of the four major parasite species has distinguishing characteristics. Two sorts of blood film are traditionally used. Thin films are similar to usual blood films and allow species identification because the parasite's appearance is best preserved in this preparation. Thick films allow the microscopist to screen a larger volume of blood and are about eleven times more sensitive than the thin film, so picking up low levels of infection is easier on the thick film, but the appearance of the parasite is much more distorted and therefore distinguishing between the different species can be much more difficult.
From the thick film, an experienced microscopist can detect parasite levels (or parasitemia) down to as low as 0.0000001% of red blood cells. However, microscopic diagnosis can be difficult because the early trophozoites (“ring form”) of all four species look identical and it is never possible to diagnose species on the basis of a single ring form; species identification is always based on several trophozoites.
In areas where microscopy is not available, there are antigen detection tests that require only a drop of blood. OptiMAL-IT® (TCS Bio Sciences, Buckingham, England) will reliably detect falciparum down to 0.01% parasitemia and non-falciparum down to 0.1%. Paracheck-Pf® (Orchard Biomedical Systems, India) will detect parasitemias down to 0.002% but will not distinguish between falciparum and non-falciparum malaria. Parasite nucleic acids may also be detected using polymerase chain reaction. This technique is more accurate than microscopy. However, it is expensive and requires a specialized laboratory. Moreover, levels of parasitemia are not necessarily correlative with the progression of disease, particularly when the parasite is able to adhere to blood vessel walls. Limited molecular methods are available in some clinical laboratories and rapid real-time assays, for example, QT-NASBA (real-time quantitative nucleic acid sequence-based amplification) based on the polymerase chain reaction) but are only now being developed. Therefore, sensitive, low-tech diagnosis tools need to be developed for in order to detect low levels of parasitaemia in the field.
What is need are reagents and methods for the rapid and accurate detection and discrimination of various malaria causing Plasmodium species.